The excitement created by Barack Obama could result in a record turnout to the US election. Photograph: Ed Andrieski/AP

A "perfect storm" could be building for US election day on November 4 because of a combination of sky-high voter interest, new ballot machines and a shortage of poll staff, the independent Pew group warned yesterday.

The Washington-based group set out a long series of problems still facing the US despite reforms aimed at avoiding a repeat of the 2000 and 2004 debacles.

The launch of the 77-page report came as legal clashes over voter registration and hours-long queues formed outside booths set up for early voting in states across the US. Voting is now underway in 46 of the 50 states, though election day is still almost a fortnight away.

Virginia, a battleground state, said it will step up security at polling booths on November 4. Election officials fear trouble because of passions aroused by the election, by long queues, or by people being told they are not eligible to vote.

Doug Chapin, director of Pew's electionline.org, said: "People talk about meltdown. It is over-optimistic to think that 130 million people can vote and something does not go wrong ... We have spent eight years sorting the plumbing, but on November 4 we are going to crank up the system."

The excitement created by Barack Obama could result in a record turnout, with African-Americans and young voters, both previously less likely to vote, predicted to cast ballots in large numbers this time round. New voters are registering in record numbers in almost every state.

Thousands of lawyers are being recruited by Obama and John McCain to police polling booths, offering advice to supporters denied the vote or challenging the eligibility of rivals.

Pew is one of the most respected research organisations in the US, and publishes annual updates on the state of the election process.

Yesterday's report, Election preview 2008: what if we had an election and everyone came?, says: "Eight years after the uncovered problems in the 2000 election and more than five years after the creation of the Help America Vote Act of 2002, millions of Americans will head to the polls on November 4 in what many are predicting will be the highest-turnout election in recent memory.

"Like the infamous Nor'easter that sank the Andrea Gail, another perfect storm may be brewing, only this one has the potential to combine a record turnout with an insufficient number of poll workers and a voting system still in flux."

Election officials are struggling in some places to recruit the tens of thousands of extra staff that will be needed.

Another problem for election officials is the electronic voting systems introduced in many states after the "hanging chads" controversy in Florida in 2000.

The report notes that voting machines bought only six years ago have been replaced in Florida, California and other states after officials and Congress became concerned about security and reliability.

Some states and counties have returned to paper, but with optical scanners that should theoretically allow for faster counting. This amounts to the third change since 2000.

One of the biggest flashpoints is voter registration, particularly in states such as Ohio, Indiana, Florida, Pennsylvania, Georgia and Missouri.

Chapin said there is fierce litigation in Indiana and Georgia, where new rules require voters to show photo IDs. Republicans claim this it aimed at preventing vote fraud, while Democrats argue it is a form of voter suppression.

The report identifies 11 states where there could be problems on election day, along with Washington DC: Indiana, Georgia, Florida, Ohio, Missouri, Pennsylvania, Wisconsin, New Mexico, Nevada, Colorado and Virginia.

Officials are encouraging early voting and absentee ballots to try to relieve the pressure on November 4.

• This article was amended on Thursday October 23 2008. We have adjusted our definition of Washington DC in the second-to-last paragraph.